Field of the Invention
The present invention relates to an energy recovery snubber for a power converter.
Description of Related Art
FIG. 1 shows a typical configuration of a flyback converter with a dissipative RCD snubber, which comprises components R2, R3, D1 and C1. The snubber is designed to absorb and dissipate energy stored in the leakage inductance of the coupled inductor (flyback transformer) L1. In a typical design, a snubber of this type will dissipate in the region of 2-4% of the throughput power of the converter, and thus reduces the efficiency of the converter by the same amount.
The primary purpose of the snubber is to limit the voltage across switch M1 so that this voltage stays within safe operating limits, and thereby prevents the energy stored in the leakage inductance of the coupled inductor causing the switch to avalanche.
FIG. 2 shows approximate operating waveforms in boundary conduction mode (BCM) for the circuit in FIG. 1. The drain-source voltage of switch M1 is clamped at a value approximately equal to the voltage on the clamp capacitor C1, while current decays in the primary winding L1a and rises in secondary winding(s) L1b. Once current has fallen to zero in the primary winding, the drain-source voltage of switch M1 will exhibit a damped oscillation back to the reflected secondary voltage, Vreflected, and current will decay at a rate proportional to the output voltage. Once current in the secondary winding(s) falls to zero, the drain-source voltage on switch M1 falls, initiating the next cycle.
FIG. 3 shows an active clamp flyback converter, which provides a known alternative to the use of RCD and other dissipative snubbers. In the active clamp flyback topology, switch M2 is controlled to conduct (ie, to be ON) when M1 is not conducting (ie, OFF) and to be OFF when switch M1 is ON. Deadtime, when both switches are OFF is typically added to allow for reducing switching losses.
In the active clamp flyback, capacitor C1 is charged to a voltage that is approximately equal to the reflected secondary voltage, Vreflected. When switch M1 is turned OFF at the end of its ON-time, a resonance takes place between the leakage inductance of the coupled inductor L1 and capacitor C1. Capacitor C1 initially charges and then starts to discharge. In the steady-state, the ampere-seconds applied to C1 over a complete switching cycle must be zero, and the resonant period is preferably longer than the converter OFF-time (ie, the OFF-time of switch M1).
FIG. 4 shows approximate operating waveforms for the circuit in FIG. 3. However, it will be appreciated that the exact waveforms will depend on the damping in the system and the amount of leakage energy being handled by the active clamp.
Ideally, the primary current will exhibit one resonant cycle during the OFF-time of the converter, as shown in FIG. 4. However, in practice, this is difficult to achieve, especially when using a controller designed to operate in boundary conduction mode (BCM). Typically, the primary current exhibits multiple resonant cycles during the OFF-time of the converter, leading to jitter on the zero-current point in the secondary circuit. The resonant current in the primary winding leads to extra losses in the primary winding of the transformer, and the impact of jitter will vary depending on the OFF-time.
Although these issues might be resolved by not operating in BCM, BCM is a preferred mode of operation for low-power converters, since it results in relatively low switching losses over the entire load range when used with burst mode, valley counting, and other techniques employed in the latest controllers. Moreover, synchronous rectification of BCM flyback converters is also relatively easy to implement with a low parts count.
It is an object of the present invention to overcome the drawbacks of the prior art.